In a combined cycle power generation plant a gas turbine is associated with a heat recovery steam generator for the generation of superheated steam which is applied to a steam turbine. The steam turbine is the major source of power, although the gas turbine is concurrently used for generating electrical power. The present invention involves cooling-down of the power generation plant, in general, and more particularly it relates to an improved way of shutting down the gas turbine in such a combined cycle power generation plant. A stopping sequence may be started upon one gas turbine and its associated boiler, independently from the other gas turbines which are still operating to support the generation of steam for the steam turbine under load, or the stopping sequence may be the final step in the process of shutting down the entire plant, steam turbine included.
Before shutting down the gas turbine, the normal procedure is to reduce the load and operating levels on all other operating units, among which the steam turbine and the heat recovery steam generator are of major importance. It is desirable in the shutting down process to let the steam turbine use all the steam available from the steam generator and to shut down the afterburner and slow down the gas turbine first, thereby allowing the steam generator to cool off.
The shutting down of the gas turbine being part of the process of stopping the generation of steam altogether, the source of heat, e.g. the gas turbine, cannot be withdrawn without risk of damaging the installation. In this respect the high pressure drum in the boiler comes into consideration on two counts:
First, the supply of heat to the superheater which normally generates superheated steam, should be stopped only when the steam outputted is dry. Once the hot air from the afterburner into the stack has been stopped, the hot boiler will cool off, which will fill up the tubes of the high pressure evaporator with water, rather than with steam, while the high pressure circulating pump is working.
Secondly, at the moment the heat source is withdrawn altogether by cutting off the gas turbine, the high pressure evaporator should have stopped bubbling. This is important, otherwise the circulating pump of the boiler could be working on a column of gas, causing the bubbles to collapse and damaging the pump. Instead, there should be no steam, only water, in the tubes. By the same occurrence, the circulating pump will fill up the tubes in the HP drum, thus keeping the water at the desired level ready to start again, when necessary.
For the purpose of this disclosure, U.S. Pat. Nos. 3,924,141; 3,953,966; and 4,151,712 are hereby incorporated by reference.